The present invention relates to the field of electronic circuits, and, more particularly, to oscillators.
Many integrated circuit electronic devices use oscillators for providing timing or clock signals. In fact, as an integrated sub-circuit, the use of oscillators is ubiquitous. Although there are many circuit approaches to realizing an oscillator, the basic function is ultimately the same. Generally speaking, oscillators provide clock signals by alternating their outputs between two voltages (such as Vdd and ground, for example) at a particular frequency. One undesirable characteristic of most oscillators is that the frequency of their output signals is overly sensitive to variations in the operating voltages and also the variations in the manufacturing process parameters themselves. As supply voltage variations increase, for example, the resulting effect on the frequency may be unacceptable in many applications.
Another characteristic of most oscillators is that they require a precision voltage or current reference. Such references may not be available or may be expensive to build as a separate circuit. In addition, it may be difficult to implement such precision references in applications where very little supply current is available.
Some prior art oscillator circuits include multiple oscillators to provide additional control over output frequency. For example, two oscillators may be used in an oscillator circuit to double its resulting output frequency. One such oscillator is disclosed in U.S. Pat. No. 4,977,379 entitled xe2x80x9cDifferential Pair, Push-Push Oscillatorxe2x80x9d to Kielmeyer which contains two single oscillators that generate signals equal in frequency and 180xc2x0 out of phase. The outputs of the oscillators are connected together to provide a resulting output that is twice the frequency of the single oscillators. A stated goal of the patent is to provide a more stable output signal by forming a composite signal rather than a summation of the two spurious signals. Yet, the output frequency of such an oscillator may still be effected by variations in the supply voltage or other process parameters.
In view of the foregoing background, it is therefore an object of the invention to provide an oscillator having reduced sensitivity to supply voltage changes.
This and other objects, features, and advantages in accordance with the present invention are provided by an oscillator including a first sawtooth waveform generator for generating a first sawtooth waveform having a selectively started ramp portion and a second sawtooth waveform generator for generating a second sawtooth waveform also having a selectively started ramp portion. The oscillator further includes a controller for alternatingly controlling the first and second sawtooth waveform generators so that a transition to the ramp portion of one sawtooth waveform is based upon determining the ramp portion of the other sawtooth waveform reaching a trip point. The controller sets the trip point substantially independent of a supply voltage so that the oscillator has reduced sensitivity to supply voltage changes.
Each of the first and second sawtooth waveform generators may include at least one switching transistor and a capacitor connected thereto, and the controller may include a biasing circuit connected to the at least one switching transistor of said first and second sawtooth waveform generators. The biasing circuit may include a bias current generator generating at least one bias current related to the supply voltage.
The biasing circuit may include at least one respective biasing transistor sized, along with the at least one switching transistors of the first and second sawtooth waveform generators, to cause the transition of the first and second waveforms at the trip point. As a result, the oscillator substantially eliminates variation in output frequency over a wide range of supply voltages. Also, the oscillator does not require an external precision reference current, thereby reducing complexity of the oscillator. The at least one biasing transistor and the switching transistor may be FETs or bipolar transistors.
The controller may set the trip point adjacent a respective end of the ramp portions, and the controller may further include a hysteresis circuit for providing hysteresis in switching. Furthermore, the controller may include a set/reset flip-flop having a set input receiving the first sawtooth waveform and a reset input receiving the second sawtooth waveform. Additionally, the oscillator may include a shutdown circuit for disabling the first and second sawtooth waveform generators and the controller.
A method for generating an oscillating signal according to the invention includes generating a first sawtooth waveform having a ramp portion, generating a second sawtooth waveform also having a ramp portion, and transitioning to the ramp portion of one sawtooth waveform based upon determining the ramp portion of the other sawtooth waveform reaching a trip point.